The use of powders of mineral particles as filler in polymers is well known in the prior art, this filler making it possible to provide additional functionalities depending, in particular, on the properties of the material constituting the filler. These functionalities are, for example, increasing the thermal conductivity and/or the hardness and/or the density of the polymer. The filled polymers thus obtained are used, in particular, in numerous technical fields such as thermal interface materials, for example, thermal pastes or thermal dissipaters, or printed circuit cards.
Due to its high thermal conductivity and electrical resistivity, boron nitride (BN) powders are used in thermal management systems. Among other publications, US 2003/0073769, US 2008/0076856, WO 2008/088774 and WO 2014/136959 describe such uses. The use of a BN powder is known particularly for increasing the thermal conductivity of the polymer, which is particularly desired in thermal interface material applications, such as thermal pastes.
To improve the through plane conductivity and reduce the anisotropy of its properties, BN is used as agglomerates or spherical powder, in the range 20-500 microns. The challenge is to develop a high strength and low cost agglomerates. Two processes currently used in the industry are: (i) isostatic pressing, crushing, sieving, and firing, which suffers from low yield and high costs; and (ii) dispersing, spray-drying, and firing, which leads to spherical granules presenting very low density and relatively low strength.
There is a need for a process allowing the manufacturing of BN agglomerates presenting the following properties: yield in the range 50-500 micron above 80%, sphericity above 0.8, and improved wear resistance (i.e., low wear by attrition).
The present invention is directed to overcoming these and other deficiencies in the art.